1. Field of the Invention
The present invention relates generally to processing of waste materials, and more particularly to systems and processes for handling solid and liquid organic waste materials.
2. Description of the Prior Art
The traditional method of solid waste handling has been landfilling, the process of burying waste in a landfill. However, landfilling can cause environmentally unacceptable pollution discharges to the water and, as real estate values increase, is considered to be an unattractive use of land. Thus, current waste management strategies seek to limit the amount of refuse directed to landfills. Recycling and composting programs have become widely accepted for both commercial and residential waste to reduce the demands on landfills.
An alternative to composting for non-recyclable solid waste are refuse-to-energy plants where material is burned to create energy. Refuse-to-energy plants first process the waste by grinding and then burning the ground material. Although efforts are made to separate out hazardous materials from the waste stream, these plants have had a history of emissions and operational problems related to contaminants. The residual ash created from this burning has also, in some cases, been found to be hazardous.
Another type of organic waste material comprises the liquid discharge handled by sewage treatment plants. This material typically has high organic content in the form of biological or chemical oxygen demand (BOD & COD), and may have relatively high suspended solids (SS) content even though the material is classified as a liquid and will flow.
Anaerobic digestion presents an alternative for handling organic waste materials, both solids and liquids. The primary objective of anaerobic digestion is the production of a mixture of hydrocarbon gases (“biogas”), which may be utilized as an energy source to generate electricity and/or heat. Any solid material remaining at the completion of the anaerobic digestion process is typically disposed of by conventional landfilling or composted into a soil amendment.
Because of the high capital costs associated with anaerobic digestion equipment, and the environmental issues associated with refuse-to-energy plants, composting has become the dominant method in the United States for the management and re-use of organic waste materials generated in rural and suburban settings. The growing use of composting as a preferred alternative to disposal of organic waste material has also created some environmental problems. These problems include emissions of noxious gases and ozone pre-cursors, runoff from the compost facility, and high energy consumption during material processing. These problems may become particularly acute if the organic waste material contains large amounts of food waste or other high moisture content waste.
In the case of liquid organic waste produced by industry, this material is typically disposed of through municipal sewer systems. In some instances, because of the nature of the liquid organic waste, the producer may be required to perform certain pretreatment or complete treatment steps prior to discharging the liquid organic waste into the sewer system. In other cases, the producer is required to pay a surcharge to discharge the liquid organic waste into the sewer system. When discharged liquid organic waste reaches a sewage treatment plant, the liquid organic waste undergoes a number of processing steps before reaching the final treatment step of anaerobic digestion. Unfortunately, a significant portion of the organic content of the liquid organic waste that could otherwise be converted to energy products can be lost, depending on the type of front end processing and the type of organics in the material, before it gets to the anaerobic digestion system.
Commercial-scale composting is also subject to a variety of financial considerations including capital investment related to accommodating peak seasonal feedstock deliveries, compost process time, and controlling the timing of compost production to match the seasonal demand of the agricultural industry and other compost buyers. Further, the compost produced by these facilities is a low-value product, therefore municipalities have to pay to have the waste accepted.